With the pronounced tendency towards high quality magnetic recording media in recent years, it is strongly desired that magnetic recording discs and magnetic recording tapes having high electromagnetic conversion properties appropriate for high density recording be developed.
For the improvement in the electromagnetic conversion properties of magnetic recording media, there have so far been made a variety of attempts including use of magnetic powders having a high saturation magnetization strength, improvements in dispersability of magnetic powders, and as approaches to the improvement in magnetic layers, adoption of double-layered structure, enhancement of square ratio, coercive force, saturation magnetic flux density, and further adjustment of surface smoothness.
Covering a broad recording wave range requires that the magnetic layer be formed in double-layered structure. The expression "double-layered structure of the magnetic layer" indicates the layer structure comprising an upper magnetic layer covering short wave recording and a lower magnetic layer covering long wave recording.
Meanwhile, coating a double-layered recording medium has so far been practiced in a manner which repeats for each layer the procedure of coating and drying after that. This coating method, however, has disadvantages of poor productivity, poor capabilities of providing uniform coating, particularly uniformly thin coating, and liabilities to low electromagnetic properties. Accordingly, extrusion coating which allows simultaneous double-layer coating is being practiced.
In extrusion coating, a paint taken out of a paint reservoir is suddenly subjected to shearing force and stretching force by the edge of a coater head when it goes out of a slit and gets on a travelling support.
Since a paint for component layers such as magnetic layers is a non-Newtonian fluid, a measure with every regard to rheological characteristics is required in order to stretch a paint into a thin film in a moment from its three-dimensional liquid state.
Although the spreadability of a paint is an important technical factor in coating as described above, no study has so far been made from a viewpoint of the spreadability on coating of the component layers for recording media.